The c-myc proto-oncogene is involved in the regulation of both normal and neoplastic cell growth. Since the discovery of Max, a partner protein enabling c-Myc to bind to DNA, other gene products which interact with Max have been described. The product of the mxi1 gene is a protein which shares significant homology to c-Myc and Max, and whose expression correlates with cellular differentiation in hematopoietic cells. Mxi1 is thought to be involved in a regulatory network with Max, c-Myc and other members of the Myc protein family. In this scheme, c-Myc activates transcription and stimulates cell proliferation, and Mxi1 putatively negatively regulates these actions, promoting cellular differentiation. Mutations in or loss of the mxi1 gene could therefore prevent differentiation and enhance proliferation in the presence of normal levels of c-Myc, and thus mxi1 is a potential tumor suppressor gene. We have mapped the mxi1 gene to the distal portion of the long arm of chromosome 10, a region which is rearranged or missing in significant numbers of prostate and brain tumors. We are interested in investigating the role of mxi1 in cell growth, differentiation and neoplasia of prostate cells by setting the following specific aims: Specific Aim #1. Identification of the complete structure of the mxi1 gene: a. Isolate promoter sequences to understand mxi1 regulation b. Precisely identify exon/intron boundaries to aid in mutation analysis c. Assess for presence of specific intronic regulatory sequences Specific Aim #2. Characterization of the effects of mxi1 expression on cell growth and differentiation a. Introduce constitutive and inducible mxi1 expression vectors b. Determine the effect of mxi1 on growth and differentiation of these cells both in vitro and in vivo Specific Aim #3. Determination of the role of mxi1 in transcriptional modulation a. Determine the effect of Mxi1 on c-Myc-dependent transcription in both cellular and in vitro transcription assays b. Investigate the ability of Mxi1 to modulate transcription independent of c-Myc Specific Aim #4. Evaluation of tumor samples for Loss of Heterozygosity (LOH) and mutation at the mxi1 locus a. Screen prostate adenocarcinomas for mxi1 LOH b. Examine LOH-bearing tumors for mutations in the mxi1 gene c. Introduce expression vectors containing mxi1 mutations into cells lines to demonstrate abrogation of mxi1 tumor suppressor activity.